The reason why menstrual blood flows when a woman stands up during her period is because of the change in body position, and the menstrual blood that is stagnant in the uterine cavity will flow out with the vagina, which is normal for most of the time. Menstrual blood is not much when you are lying down and sitting, but it will increase when you get up and move around, and flow more at the moment you get up, and then return to the normal amount, as long as the amount of menstrual flow does not exceed the normal range and is not continuous, it is a normal situation, and is most often seen on the second day of menstruation. The normal menstrual flow of a woman does not exceed 80 milliliters, if the menstrual flow exceeds this range and is accompanied by obvious blood clots, which leads to dizziness, it belongs to the excessive menstruation, which is related to endometrial hyperplasia, uterine fibroids, and endometrial polyps, and needs to be dealt with accordingly.